How to identify canker sores.

- Characteristics. / Stages of formation. / Pictures.

Page Graphics | Animations.
Link to ulceration diagram.
Link to ulceration locations animation.

Dentists distinguish canker sores (recurrent minor aphthous ulcers) from other types of mouth lesions by way of:

  1. Their appearance.
  2. The location / type of tissue on which they've formed.
  3. The fact that they are recurrent.

The dentist will also quiz their patient about the signs and symptoms that they experienced (or didn't experience) both immediately preceding and during their ulcer's formation.

There's usually no medical testing (biopsy or culturing) needed to make a diagnosis.

What do they look like?

Stage 1 - (24 hours)

The earliest sign of canker sore development usually isn't visible. It involves just the sensation of tingling, burning or numbness in the area (see below) where the lesion will ultimately form.

Pictures of aphthous ulcers.

Pictures of canker sores (aphthous ulcers).

Stage 2 - (18 to 72 hours)

The earliest visible signs have now started to form. This includes the formation of a circular raised, reddened area on the skin. The lesion gradually develops a membrane coating.

Stage 3 - (1 to 14 days)

At this point, the membrane sheds off revealing a well defined ulcer as describe below. The sore may continue to grow in size for 4 to 6 days.


Diagram showing the characteristics of an aphthous ulcer.

Sores have a grayish, membrane-coated central ulceration with a surrounding red border. The skin around the lesion looks normal.


  • A single shallow ulcer that has a round or oval shape.
  • It will usually be no more than a 1/4 inch in diameter.
  • Its center will form a loosely attached yellowish or grayish membrane.
  • Its edges will be regular (not jagged) and surrounded by a reddish halo.

Other signs and symptoms.

  • The skin adjacent to the sore will appear healthy.
  • There usually is some pain associated with the lesion. The person often will want to restrict their oral activities somewhat.
  • There are no distinguishing systemic features such as a fever or malaise.

Where do canker sores form?

They usually only form on the "loose" (movable) tissues of the mouth, meaning those areas where the skin is not tightly bound to the bone underneath. (These are also the "nonkeratinized" tissues of the mouth.)

Locations in the mouth where aphthous ulcers typically form.

Canker sores form on the "loose" tissues of the mouth.

Areas that have this type of tissue covering are the:

  • Inside of the lips and cheeks.
  • The floor of the mouth.
  • The tip or underside of the tongue.
  • The soft palate.
  • The tonsillar areas.

[ While rare, it's possible for canker sores to form on the topside of the tongue or on keratinized tissues, such as the gum tissue that surrounds a person's teeth. ]

Healing time frames.

  • A canker sore will usually heal within 4 to 14 days, although some can heal in as little as 3 to 5. In most cases this healing is uneventful and results in no residual scarring. (Woo 1996) [topic references]
  • The pain associated with the lesion usually starts to subside around day 3 to 4.
  • Any ulcer that has not healed within a 2 week time frame should be evaluated by a dentist.
  • There is another, larger, kind of aphthous ulcer (see below) that characteristically does have an extended healing time frame.

Outbreak frequency.

  • Once a person has experienced an initial outbreak, the probability of recurrence is high, although the rate at which this occurs can be quite variable.
  • A rate of one outbreak every 1 to 3 months could be considered typical (encompassing 50% of those who get them). 30% of people who suffer deal with their presence on a monthly basis. (Woo 1996)
  • It's been estimated that roughly 20% of the general population gets them.
  • A person's first outbreak will typically take place between the ages of 10 and 20 years and then decrease in frequency and severity as the person ages.
  • They're most prevalent in people 10 to 40 years of age.
  • It's been suggested that women are more likely to experience them than men but not all studies have confirmed this fact.
  • A person's risk seems to be unrelated to race.
  • They are more common in people who live in North America. as opposed to those who live on other continents.
  • Non-smokers and people in upper socioeconomic groups are more likely to experience them.

Can a person have more than one at a time?

  • Outbreaks of multiple canker sores can and do occur (in contrast to the classic single-ulcer form).
  • Usually the total number of sores that will form at one time will be six or fewer. If multiple canker sores do develop they tend to be widely distributed throughout the person's mouth (as opposed to being clustered together).
  • If two sores form close to each other, they may combine into a single larger and irregularly shaped ulcer (as opposed to the classic round or oval shape).
  • The number of outbreaks that a person experiences can vary greatly. Most people will have only a few episodes a year while, at the other extreme, others will have nearly continuous outbreaks and will never be free of ulcers for an extended period of time.

Are they contagious?

  • No, canker sores are neither contagious nor infectious.

What are "major aphthous ulcers"?

Canker sores, like we have described above, are formally termed "recurrent minor aphthous ulcers." There's another type of aphthous ulcer termed "major aphthae" (also called Sutton's ulcers).

They take the form of a large deep ulceration. The healing of these lesions is typically slower and more painful than that associated with the minor variety. They may cause regional or even facial swelling.

Unlike minor aphthae, the major kind can form on all types of oral tissues (both keratinized and nonkeratinized tissues). And they can approach 1/2 inch (or more) in diameter.

Their healing usually takes between 10 and 40 days. It can, however, take months (even as new ulcers are forming). Healing is often associated with scarring.



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